1. Field of the Invention
The present invention relates to a gamma correction method, program and apparatus, an image processing method using the same, and a display apparatus.
2. Description of the Related Art
Various display apparatus such as a CRT (Cathode Ray Tube), a liquid crystal display (LCD), and a plasma display panel (PDP) have inherent gamma characteristics and chromaticity characteristics particular to each type of display.
The gamma characteristic of a display is a relationship between an input signal level and an output luminance of the display apparatus. On the other hand, the chromaticity characteristic indicates a chromaticity of three or more primary colors (for example, RGB+white) of respective displays. Since three primary colors RGB are generally used in a display, the chromaticity characteristic generally indicates the chromaticity of RGB.
These characteristics depends on the type of the display apparatus and therefore the gamma characteristics are different to a large extent and the chromaticity characteristics are also different in the respective systems of the CRT, LCD and PDP.
On the other hand, an input image signal such as a television signal and sRGB has a gamma characteristic (relationship between signal level and luminance indicated thereby) and chromaticity characteristic that are determined by predetermined specifications. Accordingly, the gamma correction process and color space conversion process are implemented for matching between the gamma characteristic and chromaticity characteristic with the video signal and display apparatus.
Television signals are usually subjected to gamma correction and chromaticity conversion matched with the gamma characteristic of a CRT, considering display to CRT. Therefore, in most television receivers using CRTs, a special gamma correction process on the CRT side has not been required. However, for display of television signals on an LCD or PDP, at least gamma correction in the general sense (process for converting the gamma characteristic of the video signal into the gamma characteristic of display apparatus) must be conducted to maintain high image quality. The gamma characteristic differs more than the color characteristic. Since difference in the gamma characteristic relates to generation of false contour and conversion in color taste of the intermediate tone, remarkable deterioration in image quality will be generated unless matching is attained by implementing gamma correction in the general sense.
The gamma correction in the general sense is assumed here as a series of processes required for matching between the gamma characteristics of the video signal and display apparatus using the “gamma correction (in the narrower sense)” for converting the linear tone-luminance characteristic to non-linear characteristic and the “inverse gamma correction” for converting the non-linear tone-luminance characteristic to linear characteristic. Thereafter, the gamma correction of narrower sense is expressed as the “positive gamma correction”, while the gamma correction in the general sense is expressed in direct as “gamma correction” in this specification.
Moreover, in order to accurately reproduce images and pictures generated conforming to the specifications of sRGB (in other words, to realize corresponding color reproduction), the color space conversion process is also required after the inverse gamma correction. In more practical, the matrix operation of 3×3 is required for the input RGB signal as disclosed in “Color Image Reproduction” (pp. 33 to 39, formula 3.11) by Tajima (reference 3).
However, in the case where the inverse gamma correction process is performed with digital process, the number of output bits must be larger than the number of input bits in order to keep the accuracy thereof. If the number of output bits is insufficient, deterioration in image quality (for example, false contour) is generated due to quantization error.
On the other hand, when the number of output bits increases, circuit scale of the inverse gamma correction circuit and the operation circuit provided in the subsequent stage becomes large in accordance with the number of bits, resulting in the problem that manufacturing cost of display apparatus becomes high.
Moreover, for corresponding color reproduction, the color space conversion is required after completion of the inverse gamma correction. In this case, the circuit scale of the color space conversion circuit also increases when the output bits generated at the time of inverse gamma correction increases.
As the related art for solving the problem that the number of bits increases, JP 10-126648 (reference 1) discloses “Input Signal Level Application Type Gamma Correction Circuit in Liquid Crystal Display Apparatus”. The reference 1 relates to a gamma correction circuit which does not generate deterioration of image quality in the conversion process using a less number of bits.
The input signal level application type gamma correction circuit in the liquid crystal display apparatus described in the reference 1 has a structure comprising an AD converter and a DA converter respectively before and after the digital gamma correction circuit and additionally comprising variable gain control amplifiers before the AD converter and after the DA converter.
The amplifying degrees of these variable gain control amplifiers are in the complementary relationship. The reference 1 well improves the tone characteristic of dark area by changing an amplifying degree in accordance with level of analog input signal. Accordingly, high precision gamma correction has been realized without increase in the number of bits.
As the other related art, an example of using a floating point to indicate the tone signal of a display apparatus is described, for example, in U.S. Pat. No. 5,528,741 (reference 2).
However, the reference 1 is based on the fact that an input signal is the analog signal. Therefore, a DA converter is further required when an input signal is the digital signal, as much complicating the circuit. The television signal (HDTV or the like) and the image signal (sRGB) are almost inputted as the digital signals and the image signal process is often conducted in digital. Therefore, it is preferable to employ a digital inverse gamma correction process circuit which can realize inverse gamma correction with sufficient accuracy without use of an AD converter and a DA converter.
The reference 2 discloses a method for effectively converting a pixel value of floating point representation into an integer value of one byte. In application of computer graphics, it is more convenient to use the floating point representation in the computation process of pixel value. However, since almost all display apparatuses process the pixel data (pixel value) used for pixel display as integers, display is impossible if the pixel data of the floating point representation is not converted to the integer representation.
Accordingly, in the reference 2, the data of floating point representation is converted to the integer representation. Moreover, the gamma correction is also conducted simultaneously at the time of conversion into the integer representation. However, this structure has realized the simplified gamma correction process but yet includes an unsolved problem that the number of bits increases after the gamma correction. Accordingly, higher accuracy and reduction in the scale of the circuits in the subsequent stage cannot be realized in the case where some arithmetic operations are executed after the gamma correction.